The spine is a complicated structure comprised of various anatomical components, which, while being flexible, provides structure and stability for the body. The spine is made up of vertebrae, each having a vertebral body of a generally cylindrical shape. Opposed surfaces of adjacent vertebral bodies are connected together and separated by intervertebral discs (or “discs”), comprised of a fibrocartilaginous material. The vertebral bodies are also connected to each other by a complex arrangement of ligaments acting together to limit excessive movement and to provide stability. A stable spine is important for preventing incapacitating pain, progressive deformity and neurological compromise.
The anatomy of the spine allows motion (translation and rotation in a positive and negative direction) to take place without much resistance but as the range of motion reaches physiological limits, the resistance to motion gradually increases, thereby bringing such motion to a gradual and controlled stop.
Intervertebral discs are highly functional and complex structures. They contain a hydrophilic protein substance that is able to attract water thereby increasing its volume. The protein, also called the nucleus pulposis, is surrounded and contained by a ligamentous structure called the annulus fibrosis (or “annulus”). The discs perform a load or weight bearing function, wherein they transmit loads from one vertebral body to the next while providing a cushion between adjacent bodies. The discs also allow movement to occur between adjacent vertebral bodies but within a limited range. In this way, the mobility (i.e., range of motion) of the spine is dependent upon the stiffness of the discs in a given segment (e.g., a pair of adjacent vertebrae) of the spine. As will be understood, such stiffness would vary depending upon the location of the spinal segment along the length of the spine. For example, a segment located in the cervical region of the spine may have a lower stiffness (i.e., greater range of motion) as compared to a segment located in the thoracic region. It will also be understood that the relative degrees of stiffness of segments would vary from one individual to another depending upon various factors that may affect the physical limits of each segment.
As will be understood, a certain amount of stiffness in spinal segments is needed for normal or symptom-free functioning. The amount of stiffness in a spinal segment can be defined as the ratio of an applied load to the induced displacement with translation or rotation. A loss of stiffness results in exaggerated movement of the associated spinal segment such as, for example, when torque is applied. From a biomechanical perspective, loss of stiffness indicates spinal instability. Exaggerated motion caused by instability or loss of physiological stiffness may result in greater stress in adjacent innervated connective tissue, and may also lead to a greater risk of nerve-root compression and irritation in the foramina.
A normally functioning intervertebral disc has the capacity to store, absorb and transmit energy applied to it. The fluid nature of the nucleus enables it to translate vertically applied pressure (axial loading) into circumferential tension in the annulus. Due to a number of factors such as age, injury, disease, etc., intervertebral discs may lose their dimensional stability and collapse, shrink, become displaced, or otherwise damaged. It is common for diseased or damaged discs to be replaced with prosthetics, or implants. One of the known methods involves replacement of a damaged disc with a spacer, which is implanted into the space originally occupied by the disc (the disc space). However, although such spacers provide the required distraction between adjacent vertebrae, they also result in or require fusion of the vertebrae. This results in essentially a solid segment and preventing any relational movement between the vertebrae within the segment. Thus, the mobility of the spinal segment is lost and additional stresses are placed on neighboring spinal segments.
Motion segment stiffness depends on the presence of a distracting force, or a force that attempts to produce positive Y-axis translation. This constant distracting force keeps not only the annular ligaments surrounding the disc space taut but also other ligaments located anteriorly and posteriorly in the spine. The motion segment then functions in a stiffer and more stable manner whereby excessive motion causing instability, pain, and neurological symptoms are reduced or eliminated. The cushioning and balancing of loads applied to the disc space are also important in attempting to restore and preserve normal functionality of the spine. Forces normally applied to the disc primarily occur from vertical loading (compression) or, in biomechanical terms, negative Y-axis translation. A device that has the inherent ability to resist or cushion negative Y-axis translation will be able to preserve disc height. However, a device that has the inherent ability to generate positive Y-axis translation (distraction) would not only be able to resist axial or Y-axis compression but would also be able to dynamically balance these loading forces acting on the disc space. Therefore, such a device would not only provide cushioning but also elastic support and balance, thereby restoring normal physiological disc function and mechanics.
Disc replacement implants that allow some movement between adjacent vertebrae have been proposed. An example of such an implant is taught in U.S. Pat. No. 6,179,874. Unfortunately, the disc replacement (i.e., implant) solutions taught in the prior art are generally deficient in that they do not take into consideration the unique physiological function of the spine. First, many of the known artificial disc implants of the prior art mainly focus on the preservation of motion without adequately addressing the restoration of normal spinal stiffness. Second, many of the known artificial disc implants are unconstrained with respect to the normal physiological range of motion in the majority of motion planes through which they move. These implants rely on existing, but in many cases diseased structures, such as degenerated facets, to limit excessive motion. This often leads to early facet joint degeneration and other collateral damage to spinal components.
The prior art also provides some intervertebral spacers that attempt to mimic the natural mobility of a spinal segment. Examples of such spacers are provided in U.S. Pat. Nos. 5,989,291 and 6,743,257, and United States Patent Application Publication No. 2005/0125063. The '291 patent teaches a spacer formed by a pair of adjacently implanted spacer devices. Each of the devices includes opposed plates separated by at least one “Belleville washer”. Although allowing for some absorption of compressive forces, the spacer of the '291 patent does not adequately allow for motion of the segment in different axes. The '257 patent teaches a generally “U” shaped spacer having a plurality of upper and lower arms protruding joined together at a fulcrum point forming the base of the “U”. The '063 publication similarly teaches a “U” shaped structure. The arms are provided with bone anchoring devices and the implant is driven into the adjacent bone structures following discectomy. The device, once implanted, allows for flexing motions in the lateral and sagittal planes. However, although being adjustable to provide a specific disc height (i.e., a desired spacing between adjacent vertebral bodies), the device does not adequately allow compressive forces to be absorbed.
Thus, there exists a need for an intervertebral implant that overcomes the deficiencies of prior art solutions. More particularly, there exists a need for a spinal implant that is able to balance the reconstruction of spinal structures by restoring motion segment stiffness while at the same time allowing for the preservation of motion, particularly translation motion along the Y (or vertical) axis.